Function vs procedure decision in SQL - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

When to use a function instead of a procedure?

Which scenario best fits the use of a SQL function rather than a procedure?

A You want to run a batch job that sends emails and logs results.

B You want to perform multiple data modifications without returning a value.

C You need to return a single value that can be used in a SELECT statement.

D You need to execute a series of commands that include transaction control.

💡 Hint

Functions return values and can be used inside queries.

Functions are designed to return a value and can be called within SQL statements like SELECT. Procedures are better for performing actions without returning values or for complex operations.

❓ query_result

intermediate

Output of calling a function in a SELECT

Given the function below, what is the output of the query SELECT calculate_tax(100);?

SQL

CREATE FUNCTION calculate_tax(amount DECIMAL) RETURNS DECIMAL DETERMINISTIC BEGIN RETURN amount * 0.1; END;

A 0.1

B 10.0

C 100.0

D Syntax error

💡 Hint

The function returns 10% of the amount.

The function multiplies the input by 0.1, so 100 * 0.1 = 10.0.

📝 Syntax

advanced

Identify the syntax error in procedure declaration

Which option contains a syntax error in the procedure declaration?

SQL

CREATE PROCEDURE update_salary(emp_id INT, new_salary DECIMAL) BEGIN UPDATE employees SET salary = new_salary WHERE id = emp_id; END;

A CREATE PROCEDURE update_salary(emp_id INT, new_salary DECIMAL) AS BEGIN UPDATE employees SET salary = new_salary WHERE id = emp_id; END;

B CREATE PROCEDURE update_salary(emp_id INT, new_salary DECIMAL) BEGIN UPDATE employees SET salary = new_salary WHERE id = emp_id; END;

C CREATE PROCEDURE update_salary(emp_id INT, new_salary DECIMAL) BEGIN UPDATE employees SET salary = new_salary WHERE id = emp_id END;

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💡 Hint

Check the keywords used in procedure declaration.

The keyword 'AS' is not used in standard SQL procedure declarations; it causes a syntax error here.

❓ optimization

advanced

Choosing between function and procedure for performance

You need to perform a calculation repeatedly inside a large SELECT query. Which choice is best for performance?

A Use a procedure because it can handle multiple statements efficiently.

B Use a function because it can modify data during the SELECT.

C Use a procedure because it avoids recalculating values.

D Use a function because it can be inlined and called within the SELECT.

💡 Hint

Functions can be called inside queries and sometimes optimized by the database engine.

Functions are designed to be called within queries and can be optimized or inlined, improving performance for repeated calculations.

🔧 Debug

expert

Why does this function cause an error when called?

Consider this function definition and call:

CREATE FUNCTION get_employee_name(emp_id INT) RETURNS VARCHAR(100) DETERMINISTIC BEGIN SELECT name FROM employees WHERE id = emp_id; END;

What error occurs when running SELECT get_employee_name(5);?

A Function does not return a value error

B Syntax error in function body

C Type mismatch error

D No error, returns employee name

💡 Hint

Functions must explicitly return a value.

The function uses SELECT but does not assign or RETURN the value explicitly, causing an error about missing return value.

Practice

1. Which of the following is true about SQL functions compared to procedures?

easy

A. Functions return a value and can be used in SELECT queries.

B. Functions perform actions but do not return any value.

C. Procedures always return a value and can be used in WHERE clauses.

D. Procedures can be used inside SELECT statements like functions.

Solution

1. Step 1: Understand the purpose of functions

   Functions are designed to return a value and can be called within queries like SELECT.

2. Step 2: Compare with procedures

   Procedures perform tasks but do not return values directly and cannot be used inside SELECT statements.

3. Final Answer:

   Functions return a value and can be used in SELECT queries. -> Option A

4. Quick Check:

   Functions return values [OK]

Hint: Functions return values; procedures perform tasks [OK]

Common Mistakes:

• Thinking procedures return values like functions
• Using procedures inside SELECT queries
• Confusing function usage with procedure usage

2. Which of the following is the correct syntax to create a SQL function that returns an integer?

easy

A. CREATE PROCEDURE getCount() BEGIN RETURN 10; END;

B. CREATE PROCEDURE getCount() RETURNS INT BEGIN RETURN 10; END;

C. CREATE FUNCTION getCount() BEGIN RETURN 10; END;

D. CREATE FUNCTION getCount() RETURNS INT BEGIN RETURN 10; END;

Solution

1. Step 1: Identify function syntax

   Functions require specifying RETURNS type and a RETURN statement inside BEGIN...END.

2. Step 2: Check options for correctness

   CREATE FUNCTION getCount() RETURNS INT BEGIN RETURN 10; END; correctly declares a function with RETURNS INT and returns a value. Others either use PROCEDURE or miss RETURNS.

3. Final Answer:

   CREATE FUNCTION getCount() RETURNS INT BEGIN RETURN 10; END; -> Option D

4. Quick Check:

   Functions need RETURNS and RETURN [OK]

Hint: Functions need RETURNS and RETURN statements [OK]

Common Mistakes:

• Using PROCEDURE keyword for functions
• Omitting RETURNS clause in function
• Using RETURN in procedures incorrectly

3. Given the following SQL code, what will be the output of SELECT calculateTax(100); if calculateTax is a function that returns 10% of the input amount?

CREATE FUNCTION calculateTax(amount INT) RETURNS INT BEGIN RETURN amount * 0.1; END;

medium

A. 100

B. 0.1

C. 10

D. Error: Cannot multiply INT by decimal

Solution

1. Step 1: Understand function logic

   The function multiplies the input amount by 0.1 to calculate 10% tax.

2. Step 2: Calculate output for input 100

   100 * 0.1 = 10, so the function returns 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   100 * 0.1 = 10 [OK]

Hint: Multiply input by 0.1 to get 10% tax [OK]

Common Mistakes:

• Confusing 0.1 with 10
• Expecting decimal output when function returns INT
• Assuming error due to type mismatch

4. You wrote this procedure to update a user's email:

CREATE PROCEDURE updateEmail(userId INT, newEmail VARCHAR(100)) BEGIN UPDATE users SET email = newEmail WHERE id = userId; END;

When you try to call it with SELECT updateEmail(1, '[e-mail]');, you get an error. What is the problem?

medium

A. Procedures cannot be called inside SELECT statements.

B. The procedure is missing a RETURN statement.

C. The UPDATE statement syntax is incorrect.

D. The procedure parameters must be declared as OUT.

Solution

1. Step 1: Understand procedure usage

   Procedures perform actions but do not return values and cannot be called inside SELECT queries.

2. Step 2: Identify error cause

   Calling a procedure with SELECT causes error because procedures are invoked with CALL, not SELECT.

3. Final Answer:

   Procedures cannot be called inside SELECT statements. -> Option A

4. Quick Check:

   Use CALL for procedures, not SELECT [OK]

Hint: Call procedures with CALL, not SELECT [OK]

Common Mistakes:

• Trying to use SELECT to call procedures
• Expecting procedures to return values
• Misunderstanding parameter directions

5. You need to create a reusable SQL routine that calculates the discount price and also logs the discount application in a table. Which approach is best?

hard

A. Use a single function to calculate discount and log the discount inside it.

B. Use a function to calculate discount and a procedure to log the discount.

C. Use a procedure to calculate discount and return the discounted price.

D. Use a procedure only to calculate discount and log the discount.

Solution

1. Step 1: Separate calculation and action

   Functions should be used for calculations returning values; procedures for actions like logging.

2. Step 2: Apply best practice

   Use a function to return the discount price, and a procedure to perform the logging task separately.

3. Final Answer:

   Use a function to calculate discount and a procedure to log the discount. -> Option B

4. Quick Check:

   Functions calculate; procedures perform tasks [OK]

Hint: Calculate with functions; log with procedures [OK]

Common Mistakes:

• Trying to do logging inside functions
• Expecting procedures to return values
• Mixing calculation and side effects in one routine